Save image as for example .png by a script
You can try to use a low-level method. The Image class is documented 
here.

The file format is stored in the attribute
Image.file_format.  

The Image class offers two methods to save images:  


save()
save_render(filepath,scene=None)


So you can probably save your image like this:

scene=bpy.context.scene
scene.render.image_settings.file_format='BMP'
image.save_render(filepath,scene)


You might try:

image.file_format='BMP'
bpy.ops.image.save_as({'area': imageEditorArea}, # emulate an imageEditor
    'INVOKE_DEFAULT', # invoke the operator
    copy=True, # make a copy of the image
    filepath=exportDir + image.name) # export it to this location